Plasma reactor

ABSTRACT

A plasma reactor ( 10 ) is provided. The plasma reactor ( 10 ) includes a reaction chamber ( 12 ) formed by a wall ( 13 ). Proximate to the first end of the reaction chamber, the plasma reactor includes a feed gas inlet ( 14 ) for creating a reverse vortex gas flow ( 16 ) in the reaction chamber. The plasma reactor ( 10 ) also includes an anode and a cathode connected to a power source for generation of an electric arc for plasma generation in said reaction chamber. The plasma reactor ( 10 ) may optionally include a movable electrode adapted for movement from a first, ignition position to a second, operational position in the reaction chamber. Also provided is a method of converting light hydrocarbons to hydrogen-rich gas, using the plasma reactor of the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 10/560,439 filed Jul. 4, 2006, which was the National StageEntry of and claims priority to PCT/U.S. 04/19589, filed Jun. 18, 2004,which further claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/480,132 filed Jun. 20, 2003, each of which is incorporatedby reference in its entirety.

TECHNICAL FIELD

The present invention relates to a plasma reactor and process for theproduction of hydrogen-rich gas from light hydrocarbons.

BACKGROUND

Improving the efficiency of energy production remains an importanttechnological goal, owing to the significant economic benefits thatresult in almost every sector of the economy. One potential method forimproving the efficiency of energy production is to provide an energyefficient method of converting light hydrocarbons to hydrogen-rich gas,to thereby increase energy production from natural gas.

Plasma fuel converters such as plasmatrons are known to reformhydrocarbons to produce hydrogen-rich gas. DC arc plasmatrons, forexample, are disclosed in U.S. Pat. Nos. 5,425,332 and 5,437,250. DC arcplasmatrons generally operate at low voltage and high current. As aresult, these plasmatrons are particularly susceptible to electrodeerosion and/or melting. DC arc plasmatrons also require relatively highpower inputs of 1 kW or more and relatively high flow rates of coolantto keep the temperature in check.

Other conventional methods for the conversion of light hydrocarbons tohydrogen-rich gas are generally energy inefficient and, as a result, inmany small-scale applications, such as the production of hydrogen forfuel cells, the cost of hydrogen gas made by these methods is notcompetitive. Thus, there is a need in the art for a more energyefficient process for the conversion of light hydrocarbons tohydrogen-rich gas.

U.S. Pat. Nos. 5,993,761 and 6,007,742 (Czernichowski et al.) describeprocesses for the conversion of light hydrocarbons to hydrogen-rich gasusing gliding arc electric discharges in the presence of oxygen and,optionally, water. In the process, two electrodes having flat sheetgeometry are arranged for arc ignition and subsequent gliding of thearc. The distance between the cathode and anode gradually increases to apoint that no longer supports the gliding arc. As a result, the glidingarc disappears at one end of the electrodes, creating pulsed plasmawherein the properties of the plasma change with time. Due to the use ofpulsed plasma, the process is relatively unstable over time. Reagentsand oxygen are preheated using an external heat source. As a result ofthe preheating of the reagents and oxygen using an external heat source,the process suffers from poor energy efficiency. A premixed feed gasincluding hydrocarbons and oxygen is introduced to the reactor locatedat the central axis of the reactor.

U.S. Pat. No. 5,887,554 (Cohn et al.) also discloses a system for theproduction of hydrogen-rich gas from light hydrocarbons. The systemincludes a plasma fuel converter for receiving hydrocarbon fuel andreforming it into hydrogen-rich gas. The plasma fuel converter can beoperated using either pulsed or non-pulsed plasma and can utilize arc orhigh frequency discharges for plasma generation. Products from theplasma fuel converter are employed to preheat air input to the fuelconverter. In one embodiment shown in FIG. 6, residence time in thereactor is increased by providing a centralized anode and a plurality ofradial cathodes to thereby cause the arc to glide towards the center ofthe reactor under the influence of gas flowing in the same direction asthe gliding arc.

U.S. Pat. No. 6,322,757 (Cohn et al.) discloses a plasma fuel converterwhich employs a centralized electrode and a conductive reactor structurewhich acts as the second electrode for creation of a plasma discharge.Reagents are fed to the reactor just below the smallest gap between theelectrodes and flow in the same direction as the gliding arc to therebyproduce hydrogen-rich gas. In alternative embodiments, air and/or fuelare preheated by counter-flow heat exchange with the products of thereforming reaction and fed to the reactor either above or just below thesmallest gap between the electrodes.

Although some improvements in the energy efficiency of plasma fuelconverters have been achieved, there remains a need for higher energyefficiencies for use of non-equilibrium low temperature plasma.

SUMMARY

Accordingly, it is an object of certain embodiments of the invention toprovide a plasma fuel converter and a process for the conversion oflight hydrocarbons to hydrogen-rich gas using a low temperature,non-equilibrium plasma.

It is another object of certain embodiments of the invention to providea plasma fuel converter and a process for the conversion of lighthydrocarbons to hydrogen-rich gas using a low temperature,non-equilibrium plasma that has a relatively high energy efficiency.

In order to achieve the above and other objects of the invention, aplasma reactor for conversion of light hydrocarbons to hydrogen-rich gasis disclosed. In a first aspect, the plasma reactor has a wall defininga reaction chamber. The plasma reactor also has an outlet. The plasmareactor has a reagent inlet fluidly connected to the reaction chamberfor creating a vortex flow in the reaction chamber. The plasma reactoralso has a first electrode and a second electrode connected to a powersource for generating a sliding arc discharge in the reaction chamber.

In another aspect of the invention, a method for plasma conversion oflight hydrocarbons to hydrogen-rich gas is provided. In the method, aplasma reactor is provided. The plasma reactor has a wall defining areaction chamber, an outlet, and a reagent inlet fluidly connected tothe reaction chamber for creating a vortex flow in the reaction chamber.The plasma reactor also has a first electrode and a second electrodeconnected to a power source for generating a sliding arc discharge inthe reaction chamber. The method includes introducing a gas selectedfrom the group consisting of one or more light hydrocarbons, oxygen, anoxygen containing gas, and mixtures thereof, into the reaction chamberin a manner that creates a vortex flow in the reaction chamber. Themethod also includes processing the light hydrocarbons using a plasmaassisted flame; and recovering hydrogen-rich gas from the reactor.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a vortex reactor in accordancewith the present invention showing the circumferential flow component ofthe first gas.

FIG. 2 is a schematic representation of a vortex reactor in accordancewith the present invention showing the axial flow component of the gasesin the reaction chamber.

FIG. 2 b is a schematic representation of a vortex reactor showing asecond swirl generator.

FIG. 3 is a schematic representation of a vortex reactor in accordancewith the present invention and having a third gas inlet.

FIG. 4 is a schematic representation of a vortex reactor in accordancewith the present invention provided with a counter-current heatexchanger.

FIG. 4 b is a schematic representation of a vortex reactor with two heatexchangers employed.

FIG. 5 is a schematic representation of an alternative embodiment of aheat exchanger which may be used in accordance with the presentinvention.

FIG. 6 is a schematic representation of a vortex reactor in accordancewith the present invention showing the movable circular ring electrodein the ignition position.

FIG. 7 a is a schematic representation of a vortex reactor in accordancewith the present invention showing the movable circular ring electrodein the reactor operating position.

FIG. 7 b is schematic representation of a vortex reactor showing acircular ring electrode supported by supporting wires.

FIG. 8 is a schematic representation of a vortex reactor in accordancewith the present invention provided with a spiral electrode.

FIG. 9 is a schematic representation of a vortex reactor in accordancewith the present invention provided with both a spiral electrode and acircular ring electrode.

FIG. 10 is a schematic representation of a vortex reactor in accordancewith the present invention provided with a circular ring electrode whichforms part of the bottom of the reactor.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention relates to a device and process for conversion oflight hydrocarbons to hydrogen-rich gas using a low temperature,non-equilibrium plasma. The term “light hydrocarbons” as used hereinrefers to C₁ to C₄ hydrocarbons, which may be saturated or unsaturated,branched or unbranched, and substituted or unsubstituted with one ormore oxygen, nitrogen, or sulfur atoms.

In general, dimensions, sizes, tolerances, parameters, shapes and otherquantities and characteristics are not and need not be exact, but may beapproximate and/or larger or smaller, as desired, reflecting tolerances,conversion factors, rounding off, measurement error and the like, andother factors known to those of skill in the art. In general, adimension, size, parameter, shape or other quantity or characteristic is“about” or “approximate” as used herein, whether or not expressly statedto be such.

Gaseous hydrocarbons and oxygen (pure oxygen or oxygen in air, or oxygenin enriched air) are the reagents in the process of the presentinvention. The conversion process consists of two steps as illustratedbelow using methane as the light hydrocarbon reagent:

CH₄+2O₂.→2H₂O+CO₂   (I)

2CH₄+CO₂+2H₂O→6H₂+3CO   (II)

Step (I) is exothermic, whereas step (II) is endothermic and tends to bethe rate-determining step.

Referring now to the drawings, wherein like reference numerals designatecorresponding structure throughout the views, and referring inparticular to FIG. 1, a schematic view of a vortex reactor 10 of thepresent invention is depicted. Vortex reactor 10 includes a reactionchamber 12. At or near the top of vortex reactor 10, there are one ormore nozzles 14 for feeding a first gas to vortex reactor 10. Nozzles 14may be located about the circumference of vortex reactor 10 and arepreferably spaced evenly about the circumference. Preferably, at leastfour nozzles 14 are employed. The first gas is introduced to reactionchamber 12 via nozzles 14 which are oriented tangential relative to wall13 of reaction chamber 12. The tangential orientation of nozzles 14imparts a circumferential velocity component 16 to the first gas as itis introduced to reaction chamber 12. The set of nozzles 14 for thefirst gas feeding will be referred to as the first swirl generator.Optionally, a second swirl generator comprising nozzles 15, shown inFIG. 2 b can be installed along the length of the chamber. Multipleswirl generators, i.e. more than two, can be installed for introductionof multiple gases as desired. Preferably all swirl generators rotate gasin the chamber in the same direction. Products leave reaction chamber 12via outlet 20 located at or near the top of reaction chamber 12.

One embodiment of the present invention employs a flange 30 with acircular opening 32 located substantially at the center of flange 30 toform a reverse vortex flow. Flange 30 is located proximate to the firstswirl generator with nozzles 14. The opening 32 in the flange 30 ispreferably circular, but may be other shapes such as pentagonal oroctagonal. The size of circular opening 32 is important to determiningthe flow pattern in reaction chamber 12. The diameter of opening 32 inflange 30 should be from about 70% up to 95% of the diameter of reactionchamber 12 to form the reverse vortex flow similar to that shown in theFIG. 2 without a considerable recirculation zone. To form the reversevortex flow with a considerable recirculation zone 110 (FIG. 2 b), thediameter of opening 32 in flange 30 should be from about 10% up to 75%of the diameter of reaction chamber 12.

The reverse vortex flow in reaction chamber 12 causes the reagents toswirl around a region of plasma and flame 80, shown, for example, inFIG. 7 a, in reaction chamber 12. This provides heating of the reagentsas they move downwardly around central core region 24. Also, the reversevortex flow increases the residence time of reactants inside reactionchamber 12. Increased residence time helps to complete the second step(II) of the conversion reaction. Large recirculation zone 110 alsopromotes completion of the conversion process especially by decreasingignition time (initiation of the first step (I) of the conversionreaction).

Reverse vortex flow in this invention means that the vortex flow hasaxial motion initially from the swirl generator to the “closed” end ofreaction chamber 12 (along wall 13 of the chamber), and then the flowturns back and moves along the axis to the “open” end of the chamber,where a swirl generator may be placed. This flow is similar to the flowinside a dust separation cyclone, or inside a natural tornado. This flowhas very interesting and useful properties. For example, gas dynamicinsulation of the central (axial) zone: walls of the chamber do not“feel” what is going on in the center. It can be cold or extremely hot(flame or plasma) in the center of reaction chamber 12. Primarily thetemperature of incoming gas defines the temperature of wall 13. For theprocess pf hydrocarbon conversion it means that the zone of combustionis separated from wall 13.

Without the reverse vortex flow, the reagents would enter reactionchamber 12 through inlet 18 and pass between the electrodes forming theplasma and leave reaction chamber 12 at a relatively high velocity, and,at least in a small reactor, incomplete conversion of the reagents ofthe conversion reaction would likely occur. The present inventionprovides an increased residence time in reaction chamber 12, by causingthe reactants to travel a greater distance in the reactor by imparting acircumferential velocity component to the reagents. Residence times canbe increased by an order of magnitude using a preferred form of thereverse vortex flow. This helps to ensure complete conversion of thereactants to products of the conversion reaction.

In the embodiment of FIG. 1, the reagents are premixed and introduced toreaction chamber 12 via nozzles 14. This creates a full volume of flamein reaction chamber 12 causing reactor wall 13 to become very hot,indicating a significant energy loss to the environment from reactor 10.As a result of this condition, care must be taken to provide safeconditions for ignition of the flame and to prevent combustion of thereagents prior to their entry into reaction chamber 12. These factorsindicate that the embodiment of FIG. 1, wherein the reagents arepremixed and fed to reaction chamber 12 via nozzles 14, is a lesspreferred embodiment of the invention. Typical inlet velocities forfeeding gas into reaction chamber 12 via nozzles 14 is from about 10 m/sto about 50 m/s.

In order to reduce heat loss to the environment and minimize the risk ofunwanted combustion outside the reactor, two separate gases or gaseousmixtures that both are non-flammable and that form together a flammablegas mixture, can be fed to reaction chamber 12 via different inlets asdepicted in FIG. 2. In the present invention, non-flammable meansnon-combustible under the conditions existing at the specified location(in this embodiment, outside the reactor). In this embodiment, a secondgas is fed from the bottom of reaction chamber 12 via gas inlet 18co-directionally with an upward axial flow component of the first gas inreaction chamber 12 accelerating this axial flow component. In thismanner, the present invention ensures a sufficiently high axial velocityin reaction chamber 12 to move a gliding arc axially upwardly for plasmacreation. The reverse vortex flow also helps to mix the first and secondgases in the reaction chamber 12.

In order to minimize the risk of unwanted combustion outside thereactor, two separate gases or gaseous mixtures that both arenon-flammable and that form together flammable gas mixture, can also befed to the reaction chamber via different swirl generators (made ofnozzles 14 and nozzles 15 as depicted in FIG. 2 b).

A preferred ratio of the tangential flow velocity to the axial flowvelocity is about 4.0. This ratio of flow velocities causes the reversevortex flow to follow approximately a 15 degree slope in reactionchamber 12. Preferably, in this embodiment, the hydrocarbon-rich feedgas is introduced to reaction chamber 12 via nozzles 14 and anoxygen-rich gas is introduced to reaction chamber 12 through inlet 18.In this manner, the flame in reaction chamber 12 can be maintained at adistance from wall 13 of reactor 10, thereby keeping the wall of reactor10 relatively cool. This is achieved as a result of the downward flow ofthe hydrocarbon-rich gas from nozzles 14 along wall 13 of reactionchamber 12, which provides insulation between the plasma and flame andreactor wall 13. In this manner, heat loss to the environment can bereduced thereby further improving the efficiency of reactor 10. However,it is also possible to achieve acceptable results by feeding thehydrocarbon-rich feed gas to reaction chamber 12 via inlet 18 and theoxygen-rich gas via nozzles 14.

Referring to FIG. 3, there is shown another embodiment of reactor 10 ofthe present invention which further includes a third inlet 26 at the topof reaction chamber 12 for introduction of a third gas to reactionchamber 12. The third gas may be employed, as necessary, to assist theflame in t reaction chamber 12. Preferably, the third gas is oxygen-richgas.

In another embodiment of the invention shown in FIG. 4, a heat exchanger40 is employed to preheat the at least one feed gas for reactor 10.Preferably, when employing two or more inlets to feed gas to reactor 10,at least two of the feed gases are preheated in heat exchanger 40. Morepreferably, both the hydrocarbon-rich gas fed via nozzles 14 and theoxygen-rich gas fed via inlet 18 are preheated in heat exchanger 40.Also, it is preferred to preheat the feed gases by counter-current heatexchange with the product stream from reactor 10 as shown in FIG. 4.This reduces the amount of energy input to the system for preheating thefeed gases, and cools the product stream, which is also desirable in theprocess of the invention.

FIG. 4 shows reactor 10, provided with a wall 13, nozzles 14, inlet 18and a product outlet 20. Product stream 50 is fed from product outlet 20to inlet 42 at a first end of heat exchanger 40, through heat exchanger40 to product outlet 43 of heat exchanger 40. Product stream 50 leavesheat exchanger 40 as a hydrogen-rich cooled gas. At least one feed gasis fed to inlets 44, 46 located at a second end of the heat exchanger 40for counter-current heat exchange with product stream 50. In theembodiment of FIG. 4, first feed gas stream 52 is fed to inlet 44 ofheat exchanger 40 and leaves heat exchanger 40 via first gas outlet 45,whereupon first feed gas stream 52 is fed to nozzles 14 of reactor 10.Second feed gas stream 54 is fed to inlet 46 of heat exchanger 40, andleaves heat exchanger 40 via second gas outlet 47, whereupon second feedgas stream 54 is fed to inlet 18 of reactor 10.

In order to increase the heat exchange capacity of heat exchanger 40,heat exchanger 40 may be filled with a heat conducting material, such asnickel pellets 48. Other suitable heat conducting materials may beemployed, though it is preferable to use nickel-based metals as the heatconducting material. In a more preferred embodiment, heat exchanger 40is partially filled with a heat conducting material, such as nickelpellets 48, as shown in FIG. 5. The remaining, unfilled portion 49 ofheat exchanger 40 may be left as empty space. In a preferred embodiment,about half of the volume of heat exchanger 40 is filled withheat-conducting material. This serves to increase the residence time ofintermediate products of product stream 50 in heat exchanger 40 tothereby improve conversion of the intermediate products to the finalproducts via step (II) of the reaction given above. In this manner,significant conversion of intermediate products to final products can berealized in heat exchanger 40.

In another embodiment of the invention shown in FIG. 4 b, two or moreheat exchangers, 40 a and 40 b, are employed to preheat the feed gasesseparately to desirable temperatures. Preferably, when employing one ormore inlets to feed pure hydrocarbon gas to reactor 10, this purehydrocarbon gas should not be preheated to the temperature higher thanthe decomposition temperature (gaseous hydrocarbons decompose under thehigh temperature conditions to soot and hydrogen, for example formethane this decomposition start temperature is about 450.degree. C.).It is preferred to preheat the feed gases by counter-current heatexchange with the product stream from reactor 10, and also to preheatoxygen-rich gas to higher temperature as shown in FIG. 4 b.

In FIG. 4 b, reactor 10 is provided with a wall 13, nozzles 14, inlet 18and a product outlet 20. Product stream 50 is fed from product outlet 20to inlet 42 a at a first end of heat exchanger 40 a, through heatexchanger 40 a to product outlet 43 a of heat exchanger 40 a. Productstream 50 then enters inlet 42 b at a first end of heat exchanger 40 b,passes through heat exchanger 40 b to product outlet 43 b of heatexchanger 40 b. Product stream 50 leaves heat exchanger 40 b as ahydrogen-rich, cooled gas. At least one feed gas is fed to inlets 44, 46located at the second ends of heat exchangers 40 b and 40 a,respectively, for counter-current heat exchange with product stream 50.In the embodiment of FIG. 4 b, first feed gas stream 52 is fed to inlet44 of heat exchanger 40 b and leaves heat exchanger 40 b via first gasoutlet 45, whereupon first feed gas stream 52 is fed to nozzles 14 ofreactor 10. Second feed gas stream 54 is fed to inlet 46 of heatexchanger 40 a, and leaves heat exchanger 40 a via second gas outlet 47,whereupon second feed gas stream 54 is fed to inlet 18 of reactor 10.

If it is necessary to preheat the hydrocarbon-rich feed gas to thetemperature higher than decomposition temperature, it is necessary todilute the hydrocarbon gas with oxygen-rich gas, but this dilutionshould not result in formation of flammable mixture in feed gas stream.

The reactor of the present invention employs a plasma-assisted flame(PAF) in reaction chamber 12. The PAF is produced by preheating reactionchamber 12 and the heat exchanger(s) with an inert gas such as nitrogen,or with a lean (leaner than the mixture of reagents for conversion)combustion mixture, and replacing the preheating gas with the feed gaseswhich provide the reagents for the reactions (I) and (II). As thereagents mix in reaction chamber 12, a flammable state is producedthereby resulting in the appearance of a flame in reaction chamber 12.Finally, the oxygen concentration in reaction chamber 12 is reduced to alow level, which is at least sufficient to maintain a stable flame andto avoid soot formation. The oxygen concentration in reaction chamber 12can alternatively be maintained at a level which provides astoichiometric amount of oxygen for the reactions (I) and (II), as longas the flame is stable at this concentration. Thus, in a preferredembodiment, the number of oxygen atoms [O] in the sum of all feed gasesthat come to reaction chamber 12 is at least as large as the number ofcarbon atoms [C] in the same sum of all feed gases coming to reactionchamber 12, as long as the flame is stable at this oxygen-rich gas feed.If the flame is stable using a stoichiometric concentration of oxygen([O]/[C]=1), part of oxygen atoms can be fed to the reactor in the formof water vapor to produce more hydrogen via the reaction:

CH₄+H₂O→CO+3H₂

Using sliding arc plasma, as in the present invention, the soot-lessflame can be maintained even with combinations of reactants which wouldnormally be outside the limit of flammability or which can burn onlywith soot production, hence the term “plasma assisted flame” (PAF)appears. The PAF provides fast conversion of reagents to intermediateproducts, while keeping the energy input to the reactor at an efficientlevel (preferably less than 2% of the total chemical energy of thehydrocarbon gas), since the PAF consumes less electrical energy thansliding arc plasma alone. The PAF also permits the use of lowerconcentrations of oxygen in reaction chamber 12 to maintain thesoot-less flame. This is desirable since lower oxygen concentrationstend to result in greater hydrogen production by minimizing the amountof water generated in reaction chamber 12 by reaction of oxygen withlight hydrocarbons.

In one embodiment of the invention, the present invention utilizes aconstant distance between electrodes to maintain a stable sliding arc inorder to avoid the production of pulsed plasma, wherein the propertiesof the plasma constantly change with time. By maintaining the slidingarc with a constant distance between the electrodes, stable plasma isobtained and the properties of the plasma do not change significantlywith time.

The stable sliding arc can be obtained, for example, using electrodes asshown in FIG. 6. In FIG. 6, a first electrode is provided in reactionchamber 12 in the form of a circular ring electrode 60, supported bysupporting wires 62 and connected to a power supply 64 via an electricalconnection 66. A second electrode 70 is preferably located in an upperportion of reaction chamber 12.

Circular ring electrode 60 is mounted, via supporting wires 62 on amovable mount 68 for substantially vertical movement in reaction chamber12. Movable mount 68 is actuatable from outside reactor 10 to permitadjustment of the distance between circular ring electrode 60 and secondelectrode 70. This arrangement permits circular ring electrode 60 to bepositioned a first, minimum distance 69 from second electrode 70 forignition of the sliding arc. Once the sliding arc is ignited, circularring electrode 60 is moved vertically downwardly using movable mount 68to position circular ring electrode 60 at a second, greater distancefrom second electrode 70, as shown in FIG. 7. In this manner, a shortdistance between circular ring electrode 60 and second electrode 70 canbe provided for ignition, and a longer distance between circular ringelectrode 60 and second electrode 70 can be provided for operation ofreactor 10. The ability to adjust the distance between the electrodesalso allows the optimization of the sliding arc plasma generation in treaction chamber 12 by selection of the optimal distance between theelectrodes for reactor operation.

Power consumption per unit length of the sliding arc for a fixed currentis constant, and electrode spot energy is constant. Thus, by increasingthe distance between circular ring electrode 60 and second electrode 70,the power consumption in reaction chamber 12 can be substantiallyincreased without increasing the current strength provided to thereactor. As a result, the sliding arc can be operated withoutoverheating, melting, evaporation and droplet erosion of the electrodesurface at the arc point. This provides a significantly improved lifeexpectancy for the electrodes.

Circular ring electrode 60, which forms the first electrode, can beinterchanged with electrodes having other geometries. A circulargeometry, for example, is desirable for a cylindrical reaction chamber12, such as that illustrated in the drawings since this geometry willmaintain the sliding arc at a relatively constant distance from wall 13of reactor 10. Thus, for a cylindrical reaction chamber 12, circularring electrode 60 can be interchanged with, for example a flat circulardisc, not shown. Second electrode 70 can also be in the form of acircular ring electrode or flat circular disc. In a more preferredembodiment, second electrode 70 also acts as a flow restrictor and thusmay take the place of flange 30, discussed above.

Referring to FIG. 7, there is shown reactor 10 of FIG. 6 with circularring electrode 60 in position to maintain a stable sliding arc forplasma generation. As shown in FIG. 7, the combination of the gas flows,electrode geometry and reagent mixture provide a PAF 80. Reagents flowaround PAF 80 in a reverse vortex flow pattern 82, as shown. The stablesliding arc can be obtained, for example, using electrodes as shown inFIG. 7 b. In FIG. 7 b, a first electrode is provided in reaction chamber12 in the form of a circular ring electrode 60, connected to a powersupply 64. Second electrode 70 may be in the form of a circular ringelectrode or flat circular disc. In a more preferred embodiment, secondelectrode 70 also acts as a flow restrictor and thus may take the placeof flange 30. Also shown in FIG. 7 b are swirl generators comprised ofnozzles 15 and 14.

The distance between the circular ring electrode and groundedcylindrical wall of the reactor is small enough to ensure electricalbreakdown in cold gas. Once the breakdown takes place, the sliding arcis elongated and rotated by the gas flow and reaches the constantlength, which is the largest possible length.

In another embodiment shown in FIG. 8, the present invention employs aspiral electrode 90 as the cathode for providing the sliding arc. Theanode may again be a flat disc 70 or circular ring as in the previousembodiments. Spiral electrode 90 may be anchored to the reactor 10 atone end thereof by any suitable attachment mechanism 92, such as ascrew. Preferably spiral electrode 90 is of sufficient structuralrigidity to support itself within reaction chamber 12, as shown. Spiralelectrode 90 produces an arc, which slides from free end 94 of spiralelectrode 92 toward anchored end 93 of spiral electrode 90.

The movement of the sliding arc is the result of reverse vortex flow 82in reaction chamber 12. Since the sliding arc moves around, the arc spoton the surface of spiral electrode 90 continuously moves to a newlocation, thus protecting the electrode material from excessive wear ina single location. This helps provide a longer life for spiral electrode90, and to prevent overheating, melting, evaporation and/or dropleterosion of the electrode surface at the arc point. Since the length ofthe sliding arc elongates by the reverse vortex flow, the arc reachesthe maximal possible length, extinguishes and starts again once reactor10 is running. Moreover, reverse vortex flow 82 of reagents in reactionchamber 12 helps provide easy breakdown conditions for the sliding arcin reactor 10.

The shape of spiral electrode 90 can be optimized based on the flowconditions within reaction chamber 12, and the type of power supplyemployed. For example, experimental flow visualization, numericalmodeling and/or computerized flow simulation can be employed to helpdesign the optimal shape for spiral electrode 90. For the preferredshape for spiral electrode 90 the diameter of each successive spiraldecreases relative to the previous spiral, as the distance from anode 70increases. Also, it may be preferable for the bottom of the spiral toform a circular ring to provide a similar geometry to that shown belowin FIG. 9.

When a high potential, e.g. 3 kV/mm is applied across the electrodes,electrical breakdown ignites the gliding arc. The strong reverse vortexflow 82 in reaction chamber 12 forces the gliding arc to move around thelongitudinal axis 100 of the reactor 10. The arc thus elongates itselfalong spiral electrode 90 until it eventually reaches the end of spiralelectrode 90 furthest away from anode 70. Since the gliding arc ismaintained in a central zone of t reaction chamber 12 by spiralelectrode 90 as shown in FIG. 8, it is subjected to significantly lessflow disturbances than it would be subjected to if the gliding arcextended closer to wall 13 of reactor 10. Also, the area of the glidingarc is subjected to intensive convective cooling as a result of reversevortex flow 82 and the gliding arc is thermally insulated from wall 13of reactor 10 by this same reverse vortex flow 82. These factors allowthe provision of high plasma density, high power and high operatingpressures, high electron temperatures, and relatively low gastemperatures. This combination of properties allows the selectivestimulation of certain chemical processes within reactor 10, if desired.

In another embodiment of the present invention, shown in FIG. 9, acombination of a spiral electrode 90 and a circular ring electrode 60 isemployed. This embodiment combines the advantage of having the arcbetween circular ring electrode 60 and anode 70 during normal operationof reactor 10 with the ability to reignite the sliding arc withoutmoving circular ring electrode 60, if, for any reason, the arc shouldextinguish itself. Thus, in operation, the sliding arc is ignited atfree end 94 of spiral electrode 90 and moves down spiral electrode 90 asdescribed above. Once the sliding arc reaches circular ring electrode60, it is maintained between circular ring electrode 90 and anode 70.Should the arc be extinguished, it will immediately reignite at free end94 of spiral electrode 90 and the process will repeat itself. Thisarrangement adds additional stability to the plasma generation byminimizing the time that the arc is extinguished.

The arrangement shown in FIG. 9 is for the case of DC or two-phase ACpower. For three-phase AC power, multiple arrangements of electrodes asshown in FIG. 9, can be employed.

In yet another embodiment, shown in FIG. 10, a circular ring electrode60 forms part of the bottom end of reactor 10.

In yet another embodiment (not shown), spiral electrode 90 forms part ofcylindrical wall 13 of reactor 10.

It is to be understood that various features of the differentembodiments shown in the drawings may be combined with one another in avortex reactor in accordance with the present invention. For example,the various embodiments of heat exchanger 40 can be employed in any ofthe embodiments of the vortex reactor shown in the figures.

In a second aspect, the present invention relates to a method for theconversion of light hydrocarbons to hydrogen-rich gas in a vortexreactor. The method includes the steps of introducing at least one lighthydrocarbon and oxygen into a reaction chamber, subjecting at least thelight hydrocarbon feed gas to a reverse vortex flow, and converting saidlight hydrocarbons to hydrogen-rich gas with a plasma assisted flame(PAF).

In the method, the axial gas flow may be created by the steps of feedinggas in an axial direction into said reaction chamber and, optionally,accelerating said axial gas flow through a flow restriction. Thecircumferential gas flow may be created by the step of feeding gas intosaid reaction chamber in a direction tangential to a sidewall of saidreaction chamber. In order to assist in the maintenance of the PAF, athird, oxygen-rich gas stream can optionally be introduced at the top ofthe reaction chamber.

The method includes generating plasma in said reaction chamber. Plasmageneration may include the step of providing a sliding electrical arc insaid reaction chamber, as discussed above.

The methods of the present invention may employ any of the reactorsshown in the figures. In addition, each method of the present inventionmay optionally include the step of preheating one or more feed gases bycounter-current heat exchange with the product stream from the vortexreactor.

If a vortex reactor with a movable electrode is employed, the method mayfurther include the step of moving the electrode from a first, ignitionposition, to a second, operation position after ignition of the slidingarc in the reactor. In this method, operating conditions can beoptimized, for example, by varying the distance between the movableelectrode and the fixed electrode.

It is to be understood that even though numerous characteristics andadvantages of the present invention have been set forth in the foregoingdescription, together with details of the structure and function of theinvention, the disclosure is illustrative only, and changes may be madein detail, especially in matters of shape, size and arrangement of partswithin the principles of the invention to the full extent indicated bythe broad general meaning of the terms in which the appended claims areexpressed.

1. A plasma reactor, comprising: a wall defining a reaction chamber; anoutlet; a reagent inlet fluidly connected to the reaction chamber forcreating a reverse vortex flow in said reaction chamber; a firstelectrode; and a second electrode connected to a power source forgeneration of an electric discharge in the reaction chamber.
 2. Theplasma reactor of claim 1, wherein the reaction chamber is substantiallycylindrical.
 3. The plasma reactor of claim 1, wherein said reagentinlet for creating said reverse vortex flow comprises a gas supply andone or more gas inlet nozzles oriented tangentially relative to the wallof the plasma reactor.
 4. The plasma reactor of claim 3, wherein saidreactor comprises first and second ends, the reagent inlet is locatedproximate to the first end and the outlet is also located proximate tothe first end.
 5. The plasma reactor of claim 4, wherein said reactorfurther comprises a second inlet fluidly connected to the second end ofsaid reactor.
 6. The plasma reactor of claim 4, wherein the firstelectrode is located proximate to the first end of the reactor.
 7. Theplasma reactor of claim 6, wherein the second electrode is positioned atsubstantially constant distance from the first electrode duringoperation of the reactor.
 8. The plasma reactor of claim 7, wherein atleast a portion of the second electrode is positioned in the reactionchamber to create a gap between the first electrode and the secondelectrode for initiation of a plasma generating electrical discharge atsaid gap.
 9. A plasma reactor as claimed in claim 6, wherein the firstelectrode also functions as a flow restrictor to assist in thegeneration of a reverse vortex flow.
 10. A plasma reactor as claimed inclaim 9, wherein the second electrode is a spiral shaped electrode. 11.A plasma reactor as claimed in claim 10, wherein a distal end of thespiral shaped electrode, relative to the position of the firstelectrode, terminates in a circular ring shape.
 12. A plasma reactor asclaimed in claim 9, wherein the second electrode is a combination of aspiral shaped electrode and a circular ring electrode.
 13. A plasmareactor as claimed in claim 7, wherein the second electrode is a movableelectrode which can be positioned in a first position to create a gapbetween the second electrode and the first electrode for electricdischarge ignition, and in a second position, after electric dischargeignition, at a greater distance from said first electrode to provide astable plasma in said reaction chamber.
 14. A plasma reactor as claimedin claim 1, further comprising at least one heat exchanger forpreheating at least one reagent for feeding to said plasma reactor byheat exchange with at least one product from said plasma reactor. 15.The plasma reactor of claim 1, wherein the discharge closes the distancebetween the electrodes (is attached to both electrodes).
 16. The plasmareactor of claim 15, wherein at least one discharge attachment spotmoves over the electrode surface and therefore the discharge is agliding discharge.
 17. The plasma reactor of claim 16, wherein thegliding discharge is direct current (DC) or alternative current (AC)gliding arc.
 18. The plasma reactor of claim 16, wherein the glidingdischarge is direct current (DC) or alternative current (AC) glidingnormal glow discharge.
 19. The plasma reactor of claim 1, wherein theelectric discharge is non-equilibrium plasma discharge.
 20. The plasmareactor of claim 1, wherein the plasma reactor serves for converting oflight hydrocarbons to a hydrogen-rich gas.
 21. A method for convertinglight hydrocarbons to a hydrogen-rich gas comprising the steps of:providing a plasma reactor, said plasma reactor comprising: a walldefining a reaction chamber; an outlet; a reagent inlet fluidlyconnected to the reaction chamber for creating a reverse_vortex flow insaid reaction chamber; a first electrode; and a second electrodeconnected to a power source for generation of electric discharge in thereaction chamber; introducing a gas selected from the group consistingof one or more light hydrocarbons, oxygen, an oxygen containing gas, andmixtures thereof, into said reaction chamber in a manner which creates areverse vortex flow in the reaction chamber; processing said lighthydrocarbons using a plasma assisted flame; and recovering hydrogen-richgas from said reactor.
 22. The method of claim 21, wherein said reversevortex flow is created by feeding a gas containing light hydrocarbonsinto said reaction chamber in a direction tangential to the wall of saidreaction chamber.
 23. The method of claim 21, wherein said reversevortex flow is created by feeding an oxygen-rich gas into said reactionchamber in a direction tangential to the wall of said reaction chamber.24. The method of claim 21, wherein said plasma reactor comprises firstand second ends, the reagent inlet is located proximate to the firstend, the reactor further comprises a second inlet fluidly connected tothe second end of said reactor, and wherein at least some of said gasselected from the group consisting of one or more light hydrocarbons,oxygen, an oxygen containing gas, and mixtures thereof, is provided tothe reaction chamber via the second inlet.
 25. The method of claim 24,wherein the plasma reactor comprises a movable second electrode and saidmethod further comprises the steps of igniting an electrical arc withsaid movable second electrode in a first position, and moving themovable second electrode to a second position farther from said firstelectrode than said first position for operation of said reactor. 26.The method of claim 21, wherein said gas is preheated before enteringthe plasma reactor.
 27. The method of claim 26, wherein the plasmareactor further comprising at least one heat exchanger for preheating atleast one reagent for feeding to said plasma reactor by heat exchangewith at least one product from said plasma reactor.